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Help with 215.10 (2014 NEC)

Hi,
I'm studying for my C5 Inspection Certificate and am looking for a little clarification. I purchased the Mike Holt study material and came across a question I couldn't answer (and my seasoned colleagues couldn't answer).

One of the questions asks : Ground-fault protection of equipment shall be required for a feeder disconnect if the disconnect is rated _____

A)800a/208V
B)800A/480V
C)1000A/208V
D)1000A/480V

So the answer stated in the Answer Key says the answer is "D" and refers you to section 215.10.
215.10 states : Each feeder disconnect rated 1000 amperes or more and installed on solidly grounded wye electrical system of more than 150 volts to ground, but not exceeding 600 volts phase to phase, shall be provided with ground fault protection of equipment in accordance with the provisions of 230.95.

My question is, wouldn't "C" and "D" be correct because they are both above 150V and both smaller than 600V? Am I missing something? Is it impossible for a wye electrical system to be 208v because there are 3 hot legs?

(D) is the best answer but for a 480 volt system the answers should indicate that the system is actually a solidly grounded WYE system. The choice of (D) could be a ungrounded system or a 3 wire Delta.

Thank you guys very much. So I guess my follow up question would have to be, Where can I learn why 120v goes to ground on this system? Is it like this on a delta system? Go easy on me, I'm just starting to learn this trade.

Thank you guys very much. So I guess my follow up question would have to be, Where can I learn why 120v goes to ground on this system? Is it like this on a delta system? Go easy on me, I'm just starting to learn this trade.

Here is a post from a great member here who is no longer active. It is an explanation of what you seem to asking. Give it a read.

Help with 215.10

Originally Posted by bisom

Thank you guys very much. So I guess my follow up question would have to be, Where can I learn why 120v goes to ground on this system? Is it like this on a delta system? Go easy on me, I'm just starting to learn this trade.

The answer to your follow up question (1) quoted above is illustrated in exhibit 230.27 & exhibit 230.28 (NEC 2014) if you have a hand book to view the exhibits otherwise think of it as where the OCPD is located on the Wye portion of the transformer (delta/wye) and the load conductors including the neutral (previously discussed) where the ground fault sensor is located. In general a 480Y/277V or a 600Y/437V transformer secondary was the caveat for adding distinction to prevent burndown adopted in 1971(according to the source relative to transformer protection in NEC 450-3 answers your other delta-delta question above). This will lead you to other transformer protection not asked in the original post of 215.10 since that is feeder protection and or services protection related to the 230.95 portion of your OP.

Help with 215.10

Originally Posted by nec_addicted

The answer to your follow up question (1) quoted above is illustrated in exhibit 230.27 & exhibit 230.28 (NEC 2014) if you have a hand book to view the exhibits otherwise think of it as where the OCPD is located on the Wye portion of the transformer (delta/wye) and the load conductors including the neutral (previously discussed) where the ground fault sensor is located. In general a 480Y/277V or a 600Y/347V transformer secondary was the caveat for adding distinction to prevent burndown adopted in 1971(according to the source relative to transformer protection in NEC 450-3 answers your other delta-delta question above). This will lead you to other transformer protection not asked in the original post of 215.10 since that is feeder protection and or services protection related to the 230.95 portion of your OP.

Thank you guys very much. So I guess my follow up question would have to be, Where can I learn why 120v goes to ground on this system? Is it like this on a delta system? Go easy on me, I'm just starting to learn this trade.

The 120 volts doesn't "go to ground" as you put it, but it is measured to ground in a system with an intentionally-grounded conductor, mainly because we're concerned with the possible shocking of a person who accidentally contacts an energized point.

Originally Posted by ActionDave

Here is a post from a great member here who is no longer active. It is an explanation of what you seem to asking. Give it a read.

Hi,
I'm studying for my C5 Inspection Certificate and am looking for a little clarification. I purchased the Mike Holt study material and came across a question I couldn't answer (and my seasoned colleagues couldn't answer).

One of the questions asks : Ground-fault protection of equipment shall be required for a feeder disconnect if the disconnect is rated _____

A)800a/208V
B)800A/480V
C)1000A/208V
D)1000A/480V

So the answer stated in the Answer Key says the answer is "D" and refers you to section 215.10.
215.10 states : Each feeder disconnect rated 1000 amperes or more and installed on solidly grounded wye electrical system of more than 150 volts to ground, but not exceeding 600 volts phase to phase, shall be provided with ground fault protection of equipment in accordance with the provisions of 230.95.

My question is, wouldn't "C" and "D" be correct because they are both above 150V and both smaller than 600V? Am I missing something? Is it impossible for a wye electrical system to be 208v because there are 3 hot legs?

Any help is greatly appreciated. Thank you all.

Solidly grounded wye systems will have the neutral point grounded to comply with 250.26. Line to neutral voltage will always be line to line voltage divided by 1.732. This gives you the three most common North American wye systems of 208/120, 480/277 and 600/347.

120 volts is low enough it doesn't really sustain arcing without feeding more into matter into the arc.

277 volts is much more common see an arcing situation just continue to arc until things melt down.

I suppose they just chose to word it as over 150 volts to ground to give slight leeway to exclude 208/120 systems but want to include anything over that.

600/347 volt systems are more common in Canada but you can find some in US.

Over 600 volts has traditionally been entirely different set of rules for many aspects, though that is sort of shifting more toward 1000 volts in recent code editions.